1. Field of the Invention
The present invention relates to an automatic parison feed system whereby tubular parisons e.g. of plastics material are fed onto parison pins extending vertically upwards from a horizontal parison conveyor moving within a heating furnace.
2. Description of the Prior Art
Prior to forming a container or the like by biaxially stretching a parison by blow moulding, it is well known to feed the parison vertically onto a parison pin mounted on a parison conveyor which is moving as an endless loop within a heating furnace. The parison is heated up to a predetermined stretching temperature, then a predetermined blow molding is performed. A hot-air heating furnace is usually used. For example, in the case of a biaxially stretching blow molding of a polypropylene parison, it is necessary to heat the parison up to a stretching temperature near the melting temperature of the parison, e.g. 140.degree.-170.degree. C. For effecting uniform blow molding of parisons and ensuring high accuracy (prevention of thickness deviation, etc.), the parison must be heated uniformly throughout its circumference and overall length, and to this end the temperature within the heating furnace must be maintained uniform in a predetermined, narrow range.
In a prior invention (see Japanese Patent Publication No. 43060/1976 and Utility Model Publication No. 33912/1976) filed by the present applicant and already patented, an automatic parison feed system is mounted at a lateral side exteriorly of a heating furnace; part of a parison conveyor disposed within the heating furnace extends outside the heating furnace through a pair of slit-like passages formed in the side wall of the heating furnace; parisons are vertically fed, one at a time, onto parison pins mounted on the parison conveyor outside the heating furnace; and the fed parisons are conveyed successively into the heating furnace through one of the passages by means of the parison conveyor. In such an automatic parison feed system, however, the abovementioned passages must be kept open at all times to allow the continuously moving parison conveyor to pass through them. However, if such passages are kept open at all times, the hot air within the heating furnace may escape through them, and cold air may enter the furnace thus making it difficult to maintain the furnace temperature uniform within a predetermined narrow range. For this reason, in the prior invention in question, the heated state of parison is apt to be uneven, with the result that the blow molding after heating is inconsistent or its accuracy is deteriorated. Furthermore, due to the escape of hot air and admission of cold air, the fuel consumption required for heating is increased.
As is well known, moreover, a number of parison pins are mounted on the parison conveyor and a number of parisons which have been successively fed onto those pins are conveyed around a circuit defined by an endless conveyor, within the heating furnace, the parison conveyor being moved in a continuous manner. Parisons must thus be fed onto the parison conveyor in synchronism with the continuous movement of the conveyor. But this is by no means easy.
Furthermore, in the prior invention (Japanese Utility Model Publication No. 33912/1976), successively fed parisons are held in a vertical state within a hopper which is provided with a shutter at the lower end thereof; guides are disposed circularly above a parison conveyor for rotation in synchronism with the conveyor, and when one of the guides has reached a position under the hopper, the shutter is opened to let a parison descend into the guide until the parison is received on a fixed circular bottom plate mounted below the guide frame. The inserted parisons is then moved horizontally by the guide while it is slid on the bottom plate until it falls off the bottom plate, whereupon the parison is allowed to fall onto a parison pin mounted on the parison conveyor. The parison is now pushed onto the pin by a push rod which is disposed above the parison and which is raised and lowered by means of an air cylinder. This prior invention permits an automatic feed of parisons in synchronism with a continuous movement of the parison conveyor and has proved quite effective. However, since it is necessary that the opening and closing operation of the hopper shutter and the vertical drive for the push rod should be controlled with precise timing, the structure (especially control) is complex and the entire system becomes extremely large.
In the prior invention (Japanese Utility Model Publication No. 33912/1976), moveover, parisons are fed beforehand into a feed hopper in an obliquely upwardly inclined state, and the parisons in this inclined state are conveyed successively upwards in their inclined direction by means of an endless chain having pawls for pushing the parisons until they reach the upper end of the endless chain, from where the parisons are fed to a parison loading unit continuously in a vertical state.
In this prior invention, however, the feed of parisons to the parison loading unit is effected by utilizing the motion of the parisons, which, on reaching the upper end of the endless chain fall under their own weight. Thus, the feed is effected without forcibly changing the position of parisons. As a result, however, the parison feed motion is uncertain and often causes an erroneous feed. Moreover, the feed rate is limited by the parison feed rate to the parison loading unit. Therefore, the endless chain cannot be moved at high speed and the parison fed time tends to be long and it is difficult to make a quick feed.